644 Prof. McLennan and Mr. Macallum on Intensity of 



and so make the readings at Toronto, Livorno, and Matjes- 

 iontain practically the same. 



It would seem then that 5'8 ions per c.c. per second may 

 be taken as representing approximately the ionization pro- 

 duced under normal conditions by the earth's radiation in 

 air confined at atmospheric pressure in a zinc receiver similar 

 to that used in the present investigation. 



Since Eve has shown that 1*6 ions per c.c per second is 

 all that can be estimated as being produced directly by the 

 penetrating rays coming from known radioactive substances 

 in the soil, it follows, therefore, that about 4*2 ions per c.c. 

 per second must be accounted for by the presence of other 

 types of radiation. 



From the observations made by a number of investigators, 

 it is clear that one of these types is the secondary radiation 

 produced at the walls of the zinc receiver by the penetrating 

 rays themselves. A second type which it was thought might 

 possibly be present and contribute to the ionization in the 

 receiver, was a secondary radiation produced at the surface 

 of the ground by the penetrating rays coining from the soil. 

 Some observations were taken which seem to establish the 

 existence of this type. 



The measurements which led to this conclusion were made 

 by taking readings on the ionization of the air in the zinc 

 receiver as it was gradually brought up to a high and exten- 

 sive brick wall forming part of a brick building which 

 bounded the university lawn on one of its sides. These 

 readings are recorded in Table III. and are represented by the 

 curve in the diagram. 



Table III. 



Observation Station. 



"§■" No. of 



ions per c.c. per second. 





15-1 



154 



1.0 45 



134 



16-7 



16-7 



16-5 



13*7 metres from the wall 



12-2 „ „ „ 



10-7 ,, „ „ 





4-6 ,, „ 



*3 ,, ,J M 



From the numbers given in the table it will be seen that 



